A battery pack is conventionally known in the art, for example, as disclosed in Japanese Patent Publication No. 2001-283937. According to the battery pack, multiple battery cells are electrically connected in series and multiple projecting portions (wall shaped projecting portions) are provided on opposing surfaces of the respective battery cells, each of which is formed in a flat shape and layered one another. The projecting portions, which are provided on the neighboring battery cells and opposing to each other, are brought into contact with each other, so that passages for cooling fluid are formed between the battery cells except for the projecting portions. According to another example, the multiple projecting portions (column shaped projecting portions) are dotted on the opposing surfaces of the flat shaped battery cells at predetermined intervals.
According to the above prior art, the multiple projecting portions provided on the opposing surfaces of the battery cells are so designed that the projecting portions resist binding force applied to the respective battery cells of the battery pack. As a result, since the battery cells can not sufficiently carry out a cooling function, excessive amount of the cooling fluid would become necessary. Therefore, it is a problem that an excessive driving power may become necessary for a fluid machine for generating such amount of the cooling fluid or noise may become too large.